Photon upconversion based on sensitized triplet?triplet annihilation has been observed on nanocrystalline ZrO2 films cosensitized with platinum(II) octaethylporphyrin (triplet sensitizer) and 9,10-diphenylanthracene (singlet emitter) under sunlight-like conditions (noncoherent excitation source, excitation light intensity as low as 5 mW/cm2). Time-resolved emission measurements showed a fast rise of the upconverted signal (≀10 ns), suggesting that triplet energy migration most probably occurs through a ?static? Dexter mechanism. To the best of our knowledge, this is the first observation of photon upconversion based on sensitized triplet?triplet annihilation on a sensitized mesoporous metal oxide. Implementation of similar systems in dye-sensitized solar cells would increase the maximum theoretical efficiency of these devices from 30% to over 40%.QC 2011122

Photon upconversion by sensitized triplet−triplet annihilation (UC-STTA) is a promising strategy for boosting the theoretical maximum efficiency of single threshold solar cells, in particular, dye-sensitized solar cells (DSSCs). Here, we report a substantial increase in the efficiency of UC-STTA on a nanostructured surface, using noncoherent excitation light with intensities as low as 0.5 mW cm −2 , easily achieved under sun illumination. The studied surface was a mesoporous ZrO 2 film working as a proxy system for the study of photophysics relevant to DSSCs. A wellknown UC-STTA "emitter" dye, 9,10-diphenylanthracene (DPA), was chemically modified to yield methyl 4-(10-p-tolylanthracen-9yl)benzoate (MTAB), which was chemisorbed onto ZrO 2 . The "sensitizer" dye, platinum(II) octaethylporphyrin (PtOEP), was free in butyronitrile (BuN) solution surrounding the ZrO 2 nanostructure. A rigorous oxygen removal minimized photodegradation of the dyes and enhanced triplet−triplet annihilation efficiency. The system already approaches the so-called "strong annihilation limit" at light intensities below 8 mW cm −2 . Highly efficient triplet−triplet annihilation is a requisite for the use of UC-STTA in DSSCs. Time-resolved data show that the limiting process in the UC-STTA mechanism of the present system is the dynamic triplet energy transfer step from PtOEP in solution to MTAB on the surface of ZrO 2 . This result can guide the way toward a better understanding and further efficiency improvement of UC-STTA on nanocrystalline metal oxides.

Photon upconversion by sensitized triplet−triplet annihilation (UC-STTA) is a promising strategy for breaking the Shockley−Queisser limit for efficiency of single-threshold solar cells, and in particular dye-sensitized solar cells (DSSCs). Here, we report on a heterogeneous UC system, where the annihilating dyes ("emitters") are bound to a ZrO 2 nanostructured film and the light absorbing dyes ("sensitizers") are free in solution. A comparative study of four different emitter dyes was conducted, all of them derivatives of the well-known UC-STTA emitter dye 9,10diphenylanthracene (DPA), and in every case, the sensitizer dye was platinum(II) octaethylporphyrin (PtOEP). The physical separation of emitter and sensitizer molecules in two different phases makes homogeneous triplet−triplet annihilation among sensitizers in solution a significant loss channel at high excitation intensity and low emitter surface coverage. For the studied emitter dyes, the number and type of anchor groups, and the solubility of the emitter dye in the employed solvents, are the determining factors of the UC output. The signal evolves in time and with light exposure due to emitter desorption and light-induced endoperoxide formation. These results can guide the way toward a better understanding of UC-STTA on nanocrystalline metal oxides and its development for solar energy applications.

Photon upconversion by sensitized triplet–triplet annihilation (UC-STTA) is studied in systems with triplet sensitizers and emitter molecules cochemisorbed onto nanostructured ZrO2 films. UC-STTA is a promising strategy to overcome the Shockley–Queisser efficiency limit of single-threshold solar cells. The dye-loaded mesoporous ZrO2 films studied herein allow high molecular densities and are good proxy systems for the study of photophysics relevant to dye-sensitized solar cells. Two sensitizer/emitter dye pairs are studied: platinum­(II) deuteroporphyrin IX dicarboxylic acid/4,4′-(10-(anthracene-9,10-diyl)­dibenzoic acid and platinum­(II) deuteroporphyrin IX dimethyl ester/methyl 4-(10-(p-tolyl)­anthracen-9-yl)­benzoate. Both dye pairs are closely related to the standard UC-STTA molecular pair platinum­(II) octaethylporphyrin (PtOEP)/9,10-diphenylanthracene (DPA). By chemically anchoring the upconverting dye pairs onto ZrO2 films a significant improvement in UC-STTA efficiency is achieved with respect to previously studied cophysisorbed PtOEP/DPA. Controlled variation of the sensitizer/emitter dye ratios onto the surface shows that new energy loss mechanisms appear at high sensitizer surface coverage. Spectral signatures of porphyrin aggregates suggest separate sensitizer domains form, which limits the triplet sensitization of emitter molecules. The nanosecond time scale rise and decay of the observed UC emission are likely linked to the sample stability over time; UC emission is observed 1 year after sample preparation. These are promising properties for the application of this type of system for solar energy conversion.

The gain and loss of aromaticity plays a key role in organic chemistry and in the prediction of rate-determining steps. Herein, we explore the concept of aromaticity in photoisomerization reactions. Benzannulated derivatives of the dihydroazulene-vinylheptafulvene (DHA-VHF) photoswitch were investigated using transient absorption spectroscopy and timedependent density functional theory to elucidate the effect of built-in aromaticity on the switching properties. We found that benzannulation hampered the switching ability by enhancing an already existing barrier on the excited state surface. This enhancement was found to arise from a significant loss of aromaticity in the DHA-to-VHF transition state on the excited state potential energy surface. The VHF was found to be highly aromatic on the excited state surface, showing a reversal of aromaticity compared to the ground state. The barrier was found to be dependent on the position of benzannulation, since one derivative was found to switch as fast as the nonbenzannulated molecule although with lower efficiency, whereas another derivative completely lost the ability to undergo reversible photoswitching. The findings herein provide novel principles for the design of molecular photoswitches, shedding new light on excited state aromaticity, as previous discussions have mainly considered excited state aromaticity to be beneficial to switching. Our findings show that this view must be reconsidered.[a] Mr.